A new simple technique for the determination of the diffusion length in photoconductive insulators is presented. A steady-state photocarrier grating is created by two interfering laser beams, and the magnitude of the secondary photocurrent perpendicular to the grating fringes is measured. The measurement is then repeated when the two beams are incoherent. From a determination of the two photocurrents as a function of grating period the diffusion length of the photocarriers can be obtained. The method can yield accurate results to 5% of the laser wavelength.
Shape transformations of partially capped self-assembled InAs quantum dots grown on InP are studied. Atomic force microscopy (AFM) images show large anisotropic redistribution of the island material after coverage by a 1 nm thick InP layer. The anisotropic material redistribution occurs within a few minutes and leads to a change from lens-like to elongated ring-like islands. The shape transformation is not accompanied by dot material compositional change. The formation of InAs/InP quantum rings disagrees with a previous model of InAs/GaAs ring formation that assumes that the driving force for the dot to ring transformation is the difference in surface diffusion velocity of indium and gallium atoms.
The theory underlying the steady-state photocarrier grating technique is presented, including the effect of surface recombination. Experimental results for amorphous hydrogenated silicon and semi-insulating GaAs prove that diffusion lengths ranging from 200 Å to 10 μm can be measured with an accuracy of better than 5%.
The collector multiplication in InP/Ga0.47In0.53As heterojunction bipolar transistors was found to increase with temperature, and to have a weak electric field dependence. This anomalous behavior has a profound impact on device characteristics.
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